With the recent rapid development of portable and cordless electronic equipments such as audio-visual (AV) devices and personal computers, there is an increasing demand for storage battery devices having a small size, a light weight and a high energy density as a power source to drive these electronic devices. Under these circumstances, fibrous materials having a high electrical conductivity have been noticed.
In the aforementioned application fields, there have been conventionally used carbon nanofibers (Patent Literature 1). However, the carbon nanofibers may fail to exhibit sufficient properties for storage battery devices that have been recently required to have a reduced size and a high capacity, although they having a large fiber diameter have good dispersibility in the form of a powder. In consequence, carbon nanotubes having a high electrical conductivity and a much smaller fiber diameter have been noticed. The carbon nanotubes have very excellent properties such as high strength, high electrical conductivity, high thermal conductivity, heat resistance and electromagnetic wave absorption, and therefore researches for practical utilization of the carbon nanotubes in the applications such as composite materials, semiconductor devices and conductive materials have now proceeded.
However, in order to utilize the properties of the carbon nanotubes to a maximum extent, it is necessary to loosen largely entangled aggregates of the carbon nanotubes. For this reason, the use of the carbon nanotubes in the form of a dispersion thereof has now been considered. However, it is generally known that the carbon nanotubes are hardly dispersed in a liquid. At present, there are many literatures that describe attempts for dispersing the carbon nanotubes in a liquid and forming an ink comprising the carbon nanotubes by various methods. In the Patent Literatures 2 and 3, the dispersion of the carbon nanotubes to which a dispersant is added to disperse the carbon nanotubes therein has been studied. In addition, in the Patent Literature 4, chemical modification of the carbon nanotubes themselves has been studied in order to render the carbon nanotubes easily-dispersible. However, these Patent literatures have failed to provide the carbon nanotubes that are sufficiently excellent in dispersibility and dispersion stability.
Also, dispersibility of the carbon nanotube has been improved by controlling a shape thereof. One difficulty in dispersing the carbon nanotubes in a liquid is responsible for a large length of the carbon nanotubes. Therefore, by shortening the carbon nanotubes by cutting, it is possible to deaggregate a mass of the aggregated carbon nanotubes and allow dispersion of the carbon nanotubes to proceed.
Consequently, there have been developed the carbon nanotubes that have non-continuous crystal portions in the mid of a length thereof, and tend to be cut at the joined portions therebetween. The carbon nanotubes conventionally developed have a cylindrically tubular shape, a fish bone shape (cup-laminated type), a trump shape (platelet), etc. The fish bone-shaped or trump-shaped carbon nanotubes have a lot of the non-continuous crystal portions, but they have a structure in which graphite network planes (graphene) are laminated such that the network C-axis is inclined against or perpendicular to the fiber-axis direction, so that single fibers of the carbon nanotubes tend to be deteriorated in electrical conductivity in the major fiber axis direction thereof. In addition, there have been developed carbon fibers having a cylindrical tubular shape in which several tens of so-called bell-shaped constitutional units are laminated with each other (Patent Literature 5). The carbon fibers described in the Patent Literature 5 have such a structure that the respective graphite network planes are in the form of an aggregate in which the bell-shaped constitutional units each having a closed apex portion and a barrel portion opened at a lower end thereof are laminated with a common center axis in a layer form. And at each of joined portions therebetween, the apex portion is inserted into an open end of the other constitutional unit. These fibers therefore tend to lack easily-breaking properties and tend to be still unsatisfactory in dispersibility.